The aim of this work is a description of the mechanisms involved in the transport of vitamin B12 across the cell envelope of Escherichia coli. This will focus initially on the structure and interactions of the outer membrane B12 receptor system which, as we have shown previously, also serves as the receptor system for the E colicins and bacteriophage BF23. A primary problem in understanding this B12 transport system is the mechanism of communication between the outer and inner membranes of the cell envelope. These interactions will be probed by means of affinity labeling techniques, using radioactive corrinoid compounds. It is to be hoped that these experiments will also facilitate a systematic study of the energy-coupled phase of B12 transport. Attempts will be made to identify and isolate the cofactor which is probably necessary for optimal binding of vitamin B12 and colicin El to pure solubilized receptor protein. Attempts will also be made to incorporate the purified receptor into phospholipid membrane vesicles to ascertain whether such vesicles would be capable of the facilitated diffusion transport of vitamin B12. BIBLIOGRAPHIC REFERENCES: Bradbeer, Clive. Studies on the energy-dependent phase of the transport of Vitamin B12 in Escherichia Coli. Abstracts Ann. Meeting Am. Soc. Microbiol. 174 (1975).